Sunglasses or sun glasses are a form of protective eyewear configured primarily to prevent bright sunlight and high-energy visible light from damaging or discomforting the eyes. There are sun glasses without any eyesight correcting properties and others having in addition to the eye protecting properties or vision enhancing properties the practical purpose of correcting eyesight, in particular of correcting ametropia, such as myopia or hyperopia, or of correcting presbyopia. Flip-up sunglasses add the benefits of sunglasses to corrective eyeglasses, allowing the wearer to flip up the tinted lenses for indoor use. An alternative are clip-on glasses.
The human eye shows a tremendous chromatic aberration. In particular, blue light is focused in front of the retina, yellow light is focused on the retina, and the focus of red light is located behind the retina. Therefore, in the presence of blue light a blue curtain covers the image of the objects the person is looking at, which reduces the contrast in the image.
For photopic vision, the fovea of the human eye is covered by three types of photoreceptors, the so called cones. According to the range of their sensitivity, they are called S-, M-, and L-cones (that is, corresponding to short, middle, and long wavelengths λ). FIG. 2 shows the sensitivity curves of the photoreceptors in the eye. They have a broadband characteristic. The short wavelength receptors, namely the S-cones, are sensitive to light in the wavelength λ range between 380 nm and 550 nm, that is, to blue light, only. The middle wavelength receptors, namely the M-cones, are sensitive to light in the wavelength λ range between 420 nm and 650 nm. The long wavelength receptors, namely the L-cones, are sensitive to light in the wavelength λ range between 420 nm and 700 nm. Thus, the mean wavelength receptors M and the long wavelength receptors L are not only sensitive to red and green light but also to blue light of about 450 nm.
The different types of cones are not equally distributed on the retina. There are no solely blue sensitive S-cones in the fovea. In the presence of blue light, therefore, the difference between L- and M-cone inputs is relatively reduced, and as a consequence the contrast is reduced.
FIG. 3 shows the power spectrum of a blue sky. This spectrum is dominated by blue light. Therefore, blue light is present everywhere as stray light or scattered light. Due to the sensitivity curves of the photoreceptors (cones) in the eye the stray light or the scattered light reduces the contrast. The M- and L-cones will catch blue light which will reduce the contrast because the output of both receptors is influenced by blue light. The contrast in a natural scene is perceived bigger when the difference of the input of the M- and L-cones is bigger. Therefore blue light acts against the strength of the perceived contrast.
It is well known that this effect of reduced contrast perception is in particular (but not exclusively) relevant in winter times in the presence of snow. FIG. 4 shows the spectral distribution of snow exposed to the blue sky. Lingelbach, B. and Jendrusch, G. explain in their paper entitled “Contrast Enhancing Filters in Ski Sports”, which is disclosed in the Journal of ASTM International, 2(1), 1-8 (2005) that the maximal energy in the spectral power spectrum of snow when illuminated by the blue sky is concentrated in the blue range. The blue scattered light reduces information. All colors in every natural scenery are shifted toward blue. The colors are de-saturated.
A filter reducing the perceived blue portion is known to be able to compensate for this effect. If a portion of blue light is taken out, the colors become saturated again. Such filters are standard as so-called contrast enhancing filters.
In the extreme case, all incoming blue light may be taken out. A filter having such properties is called a blue blocker. FIG. 5 shows the power transmission spectrum of a typical blue blocker. Lingelbach, B. and Jendrusch, G. explain in the place cited above that “[a]t a first glance a “blue blocker” is good for skiing due to the tremendous contrast enhancement. On the other hand, there is maximum color distortion. The subjects denied using blue blocker for skiing because they are unsuitable for skiing. The visual system seems to need blue information for peripheral vision.”
Therefore, instead of pure blue blockers so called blue attenuators are used in order to also stimulate the S-cone cells which sense blue light, only. For example, a filter of type A as disclosed in U.S. Pat. No. 5,574,517 but provided for another purpose may be used. L.c., Lingelbach and Jendrusch found out that the visual system needs blue light at least in the periphery of the visual field for peripheral vision, movement control and balance. On the short wavelength end of the visual range (λ<420 nm) the sensitivity of the M- and L-cones converges to zero. But the sensitivity of the S-cones is still very high. A filter with a power transmission of about 15% in the range of 400 nm and a lower power transmission up to 500 nm has the advantage of the huge contrast enhancement of a blue blocker, but the blue information is still there. FIG. 6 shows spectral power transmission characteristics of this kind of filter according to the prior art. Similar curves are shown in Jendrusch, G. & Lingelbach, B.: “Optimales Sehen und Filtereinsatz beim Schneesport—Mythen und Fakten.”, DOZ (Deutsche Optiker Zeitschrift) Optometrie & Fashion, 67(11), 2-7 (2012), Jendrusch, G. & Lingelbach, B.: “Farbfilter und polarisierende Filter beim Schneesport.”, Aktuelle Kontaktologie (Zeitschrift für medizinische Kontaktologie und Sportophthalmologie), 8(19), 24-28 (2012), Jendrusch, G. & Lingelbach, B.: “Zur Wirkung von Farbfiltern und polarisierenden Filtern beim Schneesport.” DOZ Optometrie & Fashion, 66 (11), 44-47 (2011) and Jendrusch, G., Ilg, A. & Lingelbach, B.: “Gut Sehen im Schnee. Optometrie”, 56(4), 5-12 (2010).
Even though such known blue attenuating filters are proven, further improvement is desired. For example skiers or snow-boarders require a better contrast perception in order to recognize irregularities in the terrain, such as hummocks or hollows even under varying light conditions and snow being directly exposed to sun light. Lingelbach and Jendrusch already showed that polarizing filters are not useful to increase contrast perception in icy terrain. Details with respect to this finding are, for example, disclosed in Jendrusch, G. & Lingelbach, B.: “Farbfilter und polarisierende Filter beim Schneesport.”, Aktuelle Kontaktologie (Zeitschrift für medizinische Kontaktologie und Sportophthalmologie), 8(19), 24-28 (2012) and Lingelbach, B.* & Jendrusch, G.: “Polarizing Filters in Ski Sports”, Journal of ASTM International, 7(10), 1-7 (2010).